Chemical vapor deposition (CVD) is a well known process in the semiconductor industry for forming thin films of materials on substrates, such as silicon wafers. In a CVD process, a substrate is placed on a substrate support inside a process chamber of a reactor. The substrate is heated, and gaseous precursors of the material to be deposited are flowed into the process chamber to form a thin layer of the material on the substrate by chemical reaction. Semiconductor devices can be formed from the deposited layers.
As semiconductor devices become smaller, requirements for the uniformity of thickness of the deposited layers have become increasingly stringent. In addition, the industry has increasingly been attempting to maximize the useable area of each substrate by reducing the size of the so-called edge exclusion zone at the edge portion of the substrate. Whereas in the past, for example, a typical industry specification might have required a thickness variation of less than 3 percent with an edge exclusion zone of no more than 10 mm on a 150 mm substrate, current industry specifications might require a thickness variation of less than 1 percent with an edge exclusion zone of no more than 3 mm on a 300 mm substrate.
The growth rate, or rate of deposit, of a layer of material in a CVD process is dependent on a number of different process parameters, including the mass flow rate of the gaseous precursors of the material into the process chamber and the temperature of the substrate. At high temperatures, the growth rate is often limited by the mass flow rate of the gaseous precursors into the process chamber. In such cases, the growth rate is said to be mass-transport limited (mass-transport regime). Accordingly, small temperature variations across the substrate have only a minimal effect on the thickness uniformity of the deposited layer.
At lower temperatures, however, the growth rate often primarily depends on temperature. In such cases, the growth rate is said to be reaction rate limited (kinetic regime). Accordingly, small temperature variations across the substrate can have a significant effect on thickness uniformity. A typical low temperature process yields a 2 percent to 3 percent variation in thickness/° C. A large number of the new reactors currently being sold are designed for these lower temperature, reaction rate limited deposition processes. Examples of such reaction rate limited processes include silicon germanium, selective silicon germanium, and silicon germanium carbon deposition processes using either silane (SiH4) or DCS at temperatures between about 500° C. and 800° C.
A variety of heat sources have been used for heating substrates in CVD reactors, including resistive, inductive, and radiative heat sources. Of these, radiative heat sources are the most common, in part because of the ease and efficiency with which they allow for temperature cycling. Radiative heat sources typically comprise a number of infrared heating lamps positioned outside of the process chamber of the reactor. Radiation from the heating lamps is transmitted through the walls of the process chamber, which typically comprise quartz, to heat a substrate supported within the chamber.
The heating lamps can be arranged in the reactor in a manner that facilitates controlling the temperature at selected locations in the process chamber. For example, in one exemplary arrangement, a first array of linear heating lamps is arranged in parallel above the substrate, and a second array of heating lamps is arranged transversely to the first array below the substrate. See, e.g., U.S. Pat. No. 4,975,651, issued Dec. 4, 1990, which is hereby incorporated by reference herein. By adjusting the power delivered to a particular lamp or group of lamps, the temperature at selected locations in the process chamber can, to some extent, be controlled.
Process engineers have been somewhat successful in using this and various other techniques to obtain a generally uniform deposition thickness over the interior portion of the substrate. FIG. 1 illustrates a typical deposition profile for a 200 mm substrate using current hardware and processing techniques. As illustrated in FIG. 1, the deposition thickness over the interior portion of the substrate is relatively uniform. However, the deposition thickness at the edge portion of the substrate is considerably less than the deposition thickness at the interior of the substrate. Conversely, the deposition thickness at the portion of the substrate just inward of the edge portion is greater than the deposition thickness at the interior portion of the substrate. This decrease in deposition thickness at the outer peripheral edge of the substrate, and increase in deposition thickness inward of the outer peripheral edge, is commonly referred to as edge rolloff.
Edge rolloff is likely due to a number of different factors, including the increased surface area to volume ratio at the edge of the substrate, which allows the edge of the substrate to cool faster than the interior of the substrate. To offset the factors contributing to edge rolloff, process engineers have increased power to the heating lamps positioned above and below the edge portion of the substrate. While this has been somewhat effective in increasing the deposition thickness at the edge portion of the substrate, however, due to the large distance between the lamps and the substrate, it has also had the undesired effect of increasing the deposition thickness inward of the edge portion of the substrate. Accordingly, the problem of edge rolloff has not been sufficiently remedied by the current hardware and processing techniques.